The invention relates to a method for coating a web being conveyed with a film forming material and an apparatus for practicing this method. More particularly, the invention relates to a method for coating a web with a film forming material including a coating liquid of organic solvent of high volatility in which the coating operation is stable over long periods of time and an apparatus for practicing this method.
The term "web" as herein used is intended to mean a belt-shaped supporting member which is made of a polyester such as polyethyleneterephthalate, polyethylene-2 and 6-napththalate, or polyolefins such as for instance polypropylene, or cellose derivatives such as cellose triacetate and cellose diacetate, or plastics such as polycarbonate, or ceramics such as glass and porcelain, or paper such as baryta paper or synthetic paper, the thickness being approximately in the range of 2 to 300.mu..
The term "coating liquid of organic solvent system of high volatility" as herein used is intended to mean one which contains a solid component which may be separately chosen according to its intended use, such as film forming materials for magnetic recording mediums or photosensitive materials. In the coating liquid, the mixing ratio of any one of the organic solvents to the solid component is extremely large, the organic solvents being used as solvents in coating the web with the coating liquid. The coating liquid is applied directly or through other coated films to the web, usually to a thickness of 5 to 500.mu. (measured before the film is dried).
The aforementioned organic solvents may, for example, be ketones such as acetone, methylethyl ketone, methylisobutyl ketone and cyclohexanone; alcohols such as methanol, ethanol, propanol and butanol; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate and glycol monoethylether acetate; glycolethers such as ether, glycoldimethylether, glycolmonoethylether and dioxane; tars (aromatic hydrocarbons) such as benzen, toluene and xylene; and chlorinated hydrocarbons such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin and dichlorobenzen.
A coating method and a coating apparatus for producing a coated film which of high quality and especially is free from stripe-like defects using a coating liquid containing organic solvent as described above (hereinafter referred to merely as "a coating liquid", when applicable) have been proposed in Japanese Laid-Open Patent Application No. 93606/1975 which discloses an extrusion-type hopper in which one lip or edge on the web entry side of the hopper is set back from the other lip or edge away from the web.
Another extrusion-type hopper has been disclosed by Japanese Laid-Open Patent Application No. 142643/1975 in which a liquid collecting recess is provided in the edge or lip in the vicinity of the coating point and downstream thereof, that is, in the vicinity of the bridge of the coating liquid which is called the meniscus, bead or ribbon and which is formed between the edge and the web.
Both of these extrusion-type hoppers are considered effective in reducing the solidified accumulations of the coating liquid which are created when the coating liquid containing an organic solvent at a high mixing ratio and high volatility evaporates while it is in contact with the solid surfaces of the hopper. Thus, an extrusion-type hopper can effectively suppress the occurrence of stripe-like defects in the longitudinal direction of the coated film, that is, in the direction of advancement of the web.
However, first-described extrusion-type hoppers are nonetheless disadvantageous in the following points. The means employed for recovering excess amounts of coating liquid and the means employed for treating the coating liquid for reuse are rather intricate. Furthermore, the techniques used for preventing solidification of the coating liquid due to evaporation during recovery are not sufficient.
On the other hand, the second-described extrusion-type hopper suffers from the following problems. When the joining point of new and old webs passes through the coating point, the meniscus or bead is greatly disturbed depending on the conditions of the joint. Furthermore, the coating liquid oozes abruptly into the aforementioned recess thus accelerating the creation of additional solidified accumulations. Therefore, it is necessary to carefully and positively join the new and old webs. That is, the new web must be satisfactorily connected to the old web at all times. For this purpose, it is required that a web accumulator of larger capacity be provided or the speed of movement of the web must be reduced.
Accordingly, the inventors have conducted intensive research on conventional extrusion-type hoppers using an organic solvent system coating liquid with a view of improving them. Furthermore, the inventors have performed experiment and analysis on slide-type hoppers which are capable of coating relatively low viscosity photosensitive silver halogenide emulsion as a thin layer, as has been disclosed in U.S. Pat. Nos. 2,761,417, 2,761,791, 2,975,754, 3,005,440 and 3,735,729, in order to improve the hoppers so as to make them capable of stably coating webs with the above-described organic solvent system coating liquid of high volatility. As a result, the inventors have produced the present invention, namely a coating method and a coating apparatus which are suitable for coating webs with the above-described organic solvent system coating liquids according to a so-called "hopper coating method" including extrusion-type hoppers and slide-type hoppers.
Accordingly, an object of this invention is to provide a novel coating method and a coating apparatus for practicing this method in which all of the above-described difficulties accompanying a conventional extrusion-type hopper have been eliminated, a conventional slide-type hopper which has been used specially for other coating liquids is made acceptable for use with the above-described coating liquids, the occurence of the above-described stripe-like defects in the coated film is prevented, and the coating operation can be continued stably for a longer period of time.